The present invention relates to a combined bright field-dark field incident light illumination apparatus for microscope instruments, including a device that can be brought into the illuminating beam as desired in order to suppress the bright field ray bundle.
In incident light illumination apparatuses which allow change as desired between bright field and dark field incident light illumination, at least part of the dark field ray bundle is kept parallel. However, in this projection to infinity, the external and internal part of the ray bundle is lost for the illumination of the dark field. This leads to a non-uniform object illumination, there being a degradation in contrast due to the inner ray bundle. In order to achieve an extensive compensation of the non-uniform object illumination it was proposed in German Offenlegungsschrift No. 2,021,784 to design the optical elements arranged in the illuminating beam to have a light transmission decreasing toward the center. However, this leads to an undesired reduction in the total brightness, which is particularly disadvantageous for large object fields.
Furthermore, it is known from German Offenlegungsschrift No. 3,100,662 to bring the dark field incident light illuminating beam to the object field via optical fibers. Although in this way it is possible largely to disregard a non-uniform object illumination, here, too, it is necessary to accept a reduction in the total brightness due to the optical fibers.
Moreover, German Offenlegungsschrift No. 2,925,407 contains a description of an incident light illumination apparatus which allows change as desired between bright field and dark field illumination. The change is carried out by a rotationally symmetrical stepped reflector which can be brought into the bright field beam, with the illuminating rays being reflected in a staircase-like manner into the dark field beams. This dark field beam is provided with a combined mirror-lens arrangement, which leads the image of the field diaphragm plane around the observation optics and into the object plane. The use of such a light ladder to change between the bright field and dark field illumination as desired can certainly achieve a higher efficiency for the light source; however, this places extreme demands on the accuracy of positioning of the light ladder, so that the projection of the plane of the field diaphragm is possible only directly into the object. With this known apparatus, larger and bright object fields cannot be illuminated uniformly in the incident light dark field.